The present invention relates to a semiconductor laser device, more particularly to a semiconductor laser device applicable for an optical information processing system, as well as an optical information processing system that uses the same.
A semiconductor laser device having a conventional Fabry-Perot type resonator structure has a larger optical power density around the facets than inside the resonator, thereby the generated heat energy causes a phenomenon of catastrophic optical damage to occur and melt the semiconductor material of the laser device. This comes to limit the upper limit value of the optical output of the laser device. In order to prevent such a phenomenon, a method has been taken; the large bandgap energy assumed at the facets of the resonator is increased, thereby forming a window structure transparent to the laser light there. A high output semiconductor laser provided with such a window structure and a method for forming the same are disclosed in the related art technical document 1: IEEE Journal of Quantum Electronics 1993, vol.29, No.6, pp.1874-1879. In the case of the semiconductor laser described in the related art technical document 1, the bandgap energy of the active layer is set largely around the facets of the resonator, thereby suppressing effectively the optical absorption around the facets and avoiding the above mentioned phenomenon of catastrophic damage even at a high optical output time (that is, when a high current is injected into the active layer). However, the semiconductor laser described in the related art technical document 1 has been confronted with problems that the laser oscillation becomes unstable in the fundamental traverse-mode when a high current is injected into the active layer, so that a kink occurs in the optical output-current characteristics and the laser light cannot follow up the waveform of the fast pulse current from a driving power supply, thereby saturating the optical output and degrading the linearity of the efficiency. Consequently, the semiconductor laser described in the related art technical document 1 has been impossible to correspond fast and linearly to the injected current waveform up to a high optical output value. The related art technical document 1 describes no idea for solving such problems.
Techniques related to the stripe optical waveguide structure for semiconductor laser devices are disclosed, for example, in the related art technical document 2: Japanese Patent Laid-Open No. Hei 5-291681 and the related art technical document 4: Japanese Patent Laid-Open No. Hei 10-154843. In the related art technical document 2, a structure with modulated stripe width is described as a structure for easier starting of self-sustained pulsating. The document 2 also describes the range for setting such a waveguide structure and the design contents. The related art technical document 3 describes how an optical waveguide structure with a modulated stripe width is employed for reducing the series resistance of the laser device, thereby suppressing heat generation. The document 3 also describes how to improve the reliability of the laser device using such an optical waveguide structure with a modulated stripe width, as well as the wavelength selectivity effective to control the longitudinal mode. However, those documents 2 and 3 do not describe anything about a method for obtaining an optical high output from the laser device and the stability of the optical output itself nor anything about the influences of the stripe structure be exerted on the fundamental traverse-mode nor anything about the guidelines of the structure for suppressing higher order modes.
On the other hand, the document 4 describes how to control the fundamental traverse-mode with respect to an optical waveguide structure with a periodically modulated stripe width, as well as the designing guidelines for such a structure. In other words, the document 4 describes a method for narrowing periodically the width of a stripe optical waveguide to be extended along the resonator, thereby making the shape of structure symmetric to the center line extended along the resonator of the stripe optical waveguide so as to secure the stability of the fundamental traverse-mode.
In the case of the conventional laser device as described in the related art technical document 1, the stripe structure employs the stripe optical waveguide whose width is fixed in the direction along the resonator and is formed in linear shape. Even in the fundamental traverse-mode, when a high current is injected in the active layer, a kink phenomenon occurs in the optical output-current characteristics due to the unstable fundamental traverse-mode. And furthermore, this makes it impossible for the laser light to follow up linearly with the driving current when data is written in memories of an optical disk and cause the memories to be deformed and the error rate to be increased in reading.
Factors causing the traverse-mode to become unstable include a hole burning phenomenon, a beam steering phenomenon, etc. The hole burning phenomenon is a phenomenon that reduces the built-in refractive index difference to the lateral direction of an active layer due to a refractive index change to occur when carriers are injected. The beam steering phenomenon is a kind of the beam instability phenomena. The phenomenon means changes of the irradiation direction and the optical output due to an interference to be caused by the coupling of the fundamental mode with the higher order (first order) mode while the laser light resonates by making a round strip in the resonator when the propagation coefficient is changed by the density of carriers, gain profile, and operation temperature in the optical waveguide.
Hereunder, description will be made for the hole burning phenomenon with reference to a semiconductor laser device provided with a stripe laser optical waveguide structure. The hole burning phenomenon is the main factor for causing the above mentioned unstable traverse mode. While a laser light is oscillated with injected carriers (a driving current) in the stripe optical waveguide structure of a semiconductor laser device, if the injected amount of carriers is increased so as to raise the optical output, the following problem arises; when high carriers are injected, the laser light intensity is increased locally in the center of the stripe optical waveguide, thus the stimulated emission of the laser light at this site is increased remarkably. Consequently, the injected carriers are much consumed due to the recombination of injected carriers, causing the center of the distributed carriers in the stripe optical waveguide to become thin just like a hole. Such a profile of carriers affects directly the refractive index profile in the emission active layer, and further the propagation of the laser light in the active layer. Such the injected carriers then cause a minus change in the refractive index difference, forming a hole-like recess in part of the refractive index profile of the emission active layer (a high refractive index area usually generated like an almost rectangular shape according to the stripe ridge) built in by an optical waveguide, etc.
In order to solve the above conventional problems of the hole burning phenomenon, measures have been taken up to now so that the refractive index difference is increased in the lateral direction of the active layer and a narrower stripe waveguide is designed, as well as a quantum well structure is employed for the active layer, thereby suppressing the reduction of the refractive index when carriers are injected.
However, these related art techniques have not been effective for solving the latter problem with the beam steering phenomenon. Hereunder, this phenomenon will be described with reference to a semiconductor laser device provided with a stripe laser optical waveguide structure.
In order to solve the beam steering phenomenon, in consideration of propagation coefficients of both fundamental mode and first-order mode, a resonator length is set so as to avoid an interference caused by the coupling of the fundamental traverse-mode with the high order mode. The method, however, has not been effective enough because of the changes of the above conditions in the optical waveguide. For a laser device manufactured with the conventional techniques, the traverse-mode goes unstable especially when the laser device is driven with a pulse current. The fundamental traverse-mode is often coupled with the first-order mode at a lower injection current, thus it is difficult to write and read data in/from memories with high reliability and respond with excellent linearity to the pulse current signal from an optical disk drive unit.
Under such the circumstances, it is a most important object of the present invention to provide a semiconductor laser device, which can suppress the hole burning phenomenon causing the fundamental traverse-mode to become unstable as described above, as well as suppress the above beam steering phenomenon, thereby obtaining excellent linear responsibility to optical outputs up to a high level, which has not been achieved by any of normal stripe structures. And furthermore, it is another object of the present invention to correct the conventional instability of the fundamental traverse-mode, which has been a problem, as well as the non-linearity of the optical output-current characteristics, thereby stabilizing the high output operation of the laser device when driven with a fast pulse current, which has not been solved completely with any of the conventional structures.
The present invention provides a semiconductor laser device, which employs a new stripe structure not realized by any of the conventional techniques, enabling high output stable operations without losing almost no fundamental characteristics and assuring the stability of the fundamental traverse-mode, which often becomes unstable when in fast driving, until a high current injection. With such the optical output-current characteristics of the laser device can correspond linearly to high outputs required for optical recording of information, which is a specification of the system apparatus. In addition, the laser device can follow up with the current pulse waveform generated from a driving power supply. It is also another object of the present invention to provide an optical information processing system employing this laser device as a light source, which is able to be driven with a fast pulse waveform and correspond to high density optical recording memories.
In order to achieve the above objects, the present invention also discloses a method for providing a semiconductor laser device that satisfies all the specifications of the characteristics required for information processing systems by inventing a new stripe structure for semiconductor laser devices so as to provide a semiconductor laser light source, which can follow up accurately with the current signal from such an optical information processing system as an optical disk drive unit, etc., as well as an optical information processing system that uses such a semiconductor laser device.
In order to achieve the above object and effect, the present invention also discloses that the newly invented stripe structure described above is characterized firstly by an optical waveguide structure provided with a modulated stripe width and inclined to the facets of the laser device, and secondly another optical waveguide structure provided with a modulated stripe width and asymmetric right and left sides in the top or cross sectional view. Consequently, the laser device and the drive unit can have the optical output-current characteristics with an excellent linearity and a fast linear responsibility even for a fast pulse current from a driving power supply of the object system.
Hereunder, the coupling of the fundamental traverse-mode with the first-order mode will be described in order to disclose the preventive measures taken in this method. If the propagation coefficients of the fundamental traverse-mode and the first-order mode are xcex20 and xcex21, the fundamental mode and the first-order mode interfere with each other when the relational expression of xcex94xcex2=nxcfx80/L is satisfied. The xcex94xcex2 means xcex94xcex2=xcex20xe2x88x92xcex21 and L is a resonator length, and n is an integer. The above relational expression indicates a condition for the mutual interference to occur between the fundamental mode and the first-order mode, that is, a condition to cause a coupling of those two modes when the phase difference between the two traverse modes is an integer multiple of xcfx80 while the laser light resonates by making a round trip in the resonator. At this time, the laser light intensity profile in the laser device becomes a winding light beam profile in the stripe area, which is none other than the beam steering phenomenon. Therefore, when the laser light goes away from the facets of the resonator, the beam is irradiated in the tangential direction of the winding light beam, thereby the radiation direction of the beam is changed from the direction vertical to the facets of the resonator. In addition, the radiated laser light output is reduced significantly. The above propagation coefficients are much controlled by various factors which affect the traverse mode, such as density of carriers, gain profile in the stripe waveguide, operation temperature conditions and injected current volume, etc. Therefore, just fixing the xcex94xcex2 of the above-mentioned relational expression and setting the resonator length so as to omit the above conditions will not be enough.
In this method, therefore, in order to avoid the increase of the gain in the first-order mode, the stripe optical waveguide is provided basically with the stripe structure increasing both optical loss inside the optical waveguide and gain difference between the first-order mode and the fundamental mode, as well as with the stripe structure being asymmetric in the right and left sides along the resonator, thereby increasing the optical loss inside the optical waveguide in the first-order mode and preventing the laser light from being closed at an integer multiple of the xcfx80 during resonating by making a round trip in the resonator so as to change the resonating condition.
More concretely, the conventional linear shape is not employed for composing the resonator of the stripe structure, but a non-linear shape is employed for composing the resonator oh the stripe structure. Then, in the top or cross sectional view, the optical waveguide is provided with the structure whose stripe shape of the right and left sides is asymmetric. Concretely, the stripe structure is formed with a periodically modulated width at both sides or one side of the stripe structure along the resonator. If the width by both sides is modulated periodically in such stripe structure, it should be avoided to compose the stripe structure with symmetric stripe shape of the right and left sides in the top or cross sectional view. Such the stripe optical waveguide can be composed firstly by inclining the center line of the stripe structure at an angle xcex8 to the direction vertical to the facets of the resonator, and secondly by using a misoriented substrate so as to be provided with asymmetric stripe shape of the right and left sides in the cross sectional view of the stripe structure, although it is provided with symmetric stripe shape in the top view. Thus, the stripe optical waveguide is set so as to be provided substantially with asymmetric stripe shape of the right and left sides. The angle xcex8 can be taken as an angle at which the laser light propagated through the stripe optical waveguide is applied obliquely to the facets of the resonator. The conditions are also set within a range of 0xc2x0 to 0.5xc2x0 including 0xc2x0 for propagating stably the laser light in the fundamental traverse-mode. More preferably, the conditions should be set a range from 0xc2x0 to 0.4xc2x0 including 0xc2x0, and furthermore preferably a range from0.1xc2x0 to0.3xc2x0. In this method, the stripe optical waveguide which is inclined is composed so that the laser light is propagated with this angle xcex8 to the facets of the resonator.
According to this method, therefore, a misoriented structure is employed as the crystal substrate for forming the semiconductor laser device. In this case, when the ridge-shaped stripe is formed in an etching process, the stripe shape becomes asymmetric in the right and left sides as shown in FIG. 4. Although the stripe optical waveguide is periodically modulated in width and composed symmetrically in its top view shown in FIG. 5, the optical waveguide becomes asymmetric in the right and left sides in its cross sectional view as shown in FIG. 4. For example, as shown in FIG. 13, a ridge-shaped structure is formed for both of the layers 17 and 18 respectively, so that the left side of the ridge-shaped stripe is inclined sharply, and the right side of it is inclined gently. The stripe structure with a periodically modulated width as shown in FIG. 14 is given only to this right side area. Consequently, the optical waveguide can also be composed asymmetrically. When the misoriented substrate is employed, the optical stripe waveguide can be composed asymmetrically, so that the above single xcex8 is not given to the facets of the resonator. For the asymmetric ridge-shaped stripe optical waveguide as shown in FIG. 13, the angle of inclination is small in the right side area and a gradual change of the refractive index difference is built in there. The stripe optical waveguide with a periodically modulated width shown in FIG. 4 can thus be designed so as to set a large optical loss in the optical waveguide effectively in the first-order mode.
According to the laser device of the present invention, therefore, the resonance of the propagated laser light in the coupling of the fundamental traverse-mode and the first order mode can be prevented. Hereunder, this effect will be described with reference to FIGS. 26A and 26B, as well as FIGS. 27A and 27B, which are all top views of the stripe structure. In the case of the conventional linear stripe structure shown in FIG. 26A and the stripe structure with a periodically modulated width as shown in FIG. 26B, the propagated laser light in the coupling of the fundamental traverse-mode with the first-order mode has to satisfy the matched condition of resonating by making a round trip in the resonator, so the laser light resonates. Under such the condition, therefore, the beam steering phenomenon occurs and this causes the output of the irradiated beam to be varied. And accordingly, a non-linear kink response appears in the optical output-current characteristics. In such a case, the conventional laser device cannot satisfy the requirement of the drive unit for the application system . Therefore, any of the conventional structure laser devices cannot cope with this problem. On the other hand, in the case of the optical waveguide structure of the present invention, which is provided with a waveguide structure inclined to the facets with a modulated stripe width as shown in FIG. 27A and an optical waveguide structure with a modulated width at only one side shown in FIG. 27B, the optical waveguide comes to have non-periodical or asymmetric sides. The propagated laser light, if the fundamental traverse-mode and the first-order mode are coupled, cannot satisfy the matched condition due to the fluctuation of phase during resonating by making a round trip of the laser light in the resonator. As shown in FIGS. 27A and 27B, even after resonating by making a round trip in the resonator, the propagated laser light cannot resonate, thus the beam steering phenomenon does not occur nor the non-linear kink does not appear in the optical output-current characteristics.
Each of those methods of the present invention has controlled stably the fundamental traverse-mode and successfully reduced occurrence of the coupling of the fundamental traverse-mode with the first-order mode significantly, because the first-order mode has the large optical loss in the optical waveguide so as to be under the cut-off condition when a high current is applied from a drive unit (a driving power supply, etc.). Thus, the technique of the laser device of the present invention can solve the above problems related to the instability of the fundamental traverse-mode when driven with a pulse current, as well as the problems related to the inability of excellent linear optical output responsibility, thereby provides a semiconductor laser device that satisfies all the requirements of optical disk systems, as well as provides a drive unit and an optical information processing system that use this semiconductor laser device as a light source respectively.
In order to achieve the above objects, first of all, the semiconductor laser device of the present invention is provided with a resonator structure for generating a laser light formed on a substrate, and a stripe optical waveguide (for controlling the traverse mode of the laser light) is provided in the resonator structure. The stripe optical waveguide is basically characterized by that the right and left sides are different in shape from each other in the cross sectional view cut at a plane vertical to the substrate surface and parallel to the facets of the resonator structure. In addition, at least one of the right and left sides has concave and convex portions in the longitudinal direction of the stripe optical waveguide in the top view thereof.
Furthermore, another laser device of the present invention is provided with a resonator structure for generating a laser light, and a stripe optical waveguide is provided in the resonator structure. The stripe optical waveguide is composed so as to have a modulated width, which is increased and reduced at least twice between the facets of the resonator structure. In addition, the center axis of the stripe waveguide in the longitudinal direction is inclined to the facets of the resonator structure at an angle of 0 to an imaginary line vertical to the facets at the shortest distance, but within 0xc2x0 less than xcex8xe2x89xa60.5xc2x0.
And furthermore, the optical information processing system of the present invention includes a light source having a resonator structure for generating a laser light formed on a substrate, an information recording medium, an optical system for applying the laser light on the information recording medium, a driving power supply for supplying a current to the light source, and a control unit for controlling the driving power supply. The light source has a stripe optical waveguide in its resonator structure. The stripe optical waveguide is basically characterized by that the right and left sides are different in shape from each other in the cross sectional view cut at a plane vertical to the substrate face and parallel to the facets of the resonator structure. In addition, concave and convex portions are formed at least at one of the right and left sides in the longitudinal direction of the stripe optical waveguide in the top view thereof.
In addition, another optical information processing system of the present invention includes a light source having a resonator structure for generating a laser light formed on a substrate, an information recording medium, an optical system for applying the laser light on the information recording medium, a driving power supply for supplying a current to the light source, and a control unit for controlling the driving power supply. The light source has a stripe optical waveguide in its resonator structure. The stripe optical waveguide is composed so as to have a modulated width, which is increased and reduced at least twice between the facets of the resonator structure. In addition, the center axis of the stripe optical waveguide in the longitudinal direction is inclined to the facets of the resonator structure at an angle of xcex8 to an imaginary line vertical to the facets at the shortest distance, but within 0xc2x0 less than xcex8xe2x89xa60.5xc2x0.
With such a configuration, it is prevented that the fundamental traverse-mode of the laser light is coupled with a high-order mode, thereby an excellent linear optical output responsibility is assured for a current injection within a range up to the driving optical output. It is also prevented that the optical output is saturated and non-linear responses are generated for the height and width of a current signal even when driven with a modulated pulse current, thereby a fast linear responsibility is obtained for the system.
In order to improve the effect of the basic configuration of the optical information processing system as described above, the present invention proposes the following four types of configurations.
In the first configuration, the stripe optical waveguide is set so as not to couple the fundamental traverse-mode of the laser light with its high order mode.
In the second configuration, the stripe optical waveguide has concave and convex portions at both the right and left sides in the longitudinal direction thereof, and the concave and convex portions at both the right and left sides are different in shape from each other between the right side and the left side. For example, the distance difference between the concave and the convex portions in the modulated stripe width, which are provided at one of those right and left sides, is set larger than that of the other.
In the third configuration, concave and convex portions, which are the same in shape, are provided on both right and left sides of the stripe optical waveguide in the longitudinal direction.
In the fourth configuration, one side of the stripe waveguide is formed linearly in the longitudinal direction.
For a further preferred configuration, the angle xcex8 should be set within 0xc2x0 less than xcex8xe2x89xa60.4xc2x0. If the angle xcex8 is set within 0.1xc2x0xe2x89xa6xcex80.3xc2x0, the above effect will further be improved. If the stripe waveguide is inclined so that the laser light is entered to the facets of the resonator at such a set angle xcex8, it is possible to obtain the expected driving optical output with an excellent linear responsibility even to a high injected current up to the driving optical output in the fundamental traverse-mode for the laser light generated by amplifying through the stripe optical waveguide structure.
On the other hand, the semiconductor laser light also includes a substrate consisting of semiconductor crystal, an emission active layer formed on the substrate, and optical waveguide layers provided on the upper and the lower surface of the emission active layer respectively. On the optical waveguide layer provided on the upper surface of the emission active layer is formed a ridge-shaped stripe structure which is extended along the longitudinal direction of the resonator structure of the laser device by processing the optical waveguide layer provided on the upper surface of the emission active layer. The present invention also recommends that the width of the stripe structure is set so as to have an effective refractive index difference in the direction in which the emission active layer crosses the longitudinal direction of the resonator structure so as to stabilize the propagation of the laser light in the emission active layer in the fundamental traverse-mode.
When the width of the stripe structure is modulated periodically or non-periodically, the laser light can correspond to high injected currents up to the driving optical output with an excellent linearity and generates a driving optical output in the fundamental traverse-mode, in which the laser light is propagated in the stripe structure and is generated by amplifying when a current is injected continuously or a pulse current is injected from the driving power supply mounted in the object system.
In this configuration, it is also possible to form the semiconductor crystal at both sides of the ridge-shaped stripe structure so as to embed the stripe structure respectively. In this case, the semiconductor crystal is given a larger bandgap energy than the bandgap energy of the emission active layer or the energy of the laser light which corresponds to the oscillation wavelength from the emission active layer. With this, an optical waveguide structure is formed in the stripe area, thereby the laser light propagation in the fundamental traverse-mode can be stabilized due to the effective refractive index difference set in the lateral direction of the active layer. Consequently, the laser light can correspond to a high injected current with an excellent linearity and generates a driving optical output in the fundamental traverse-mode, in which the laser light is propagated in the stripe structure and is generated by amplifying when a current is injected continuously or a pulse current is injected from the driving power supply mounted in the object optical information processing system.
In addition, the semiconductor crystal, which embeds both sides of the ridge-shaped stripe structure, may be composed so that its one side width are modulated periodically or both side widths are modulated non-periodically so as to compose the object optical waveguide in which the optical propagation inner loss and the refractive index difference both along the resonator in the active layer are not changed linearly nor in uniform, In other words, when both optical propagation inner loss and refractive index difference in the optical waveguide are modulated periodically or set non-periodically towards the resonator in the emission active layer, it becomes possible to stably control the fundamental traverse-mode of the laser light propagated in the optical waveguide corresponding to the refractive index difference, as well as enable fast linear correspondence to the height and width of the current signal even when driven with a modulated pulse current without saturating the optical output nor generating non-linear responses. In addition, the stripe structure for controlling the fundamental traverse-mode stably should be formed so as to propagate the laser light corresponding to the refractive index difference to be set in the lateral direction of the active layer. Then, the ridge-shaped stripe structure should be formed so that the refractive index difference mainly depends on the difference in the real part of the refractive index.
Hereunder, other examples for the semiconductor laser device of the present invention will be described one by one.
Firstly, the semiconductor laser device used here employs a crystal substrate misoriented within a range of 0 to 54.7xc2x0 (111) orientation from (100) orientation, on which the optical waveguide layer and the emission active layer are to be formed. Thus, the semiconductor laser device whose ridge-shaped stripe optical waveguide construction is formed on the misoriented crystal substrate. More preferably, the substrate orientation should be misoriented within a range of 5 to 16xc2x0 as the crystal orientation. In the case of such a semiconductor laser device, the ridge-shaped stripe structure formed on the misoriented crystal substrate should be etched, thereby the object optical waveguide structure with asymmetric stripe shape of the right and left sides should be formed, and the shape of the inclined sides forming the ridge-shaped stripe structure should be different from each other in the right and left sides. In addition, of the asymmetric stripe shape of the right and left sides which the ridge-shaped stripe structure of the above semiconductor laser device has, the width of the gently inclined ridge side, formed in the direction in which the crystal substrate is misoriented, should be modulated periodically or set non-periodically along the resonator. Because, the fundamental traverse-mode can be stabilized in the optical waveguide structure when the loss in the optical waveguide and the refractive index difference are modulated periodically or set non-periodically along the resonator.
Secondly, the semiconductor laser used here is provided with a ridge-shaped stripe structure having a resonator structure whose length is from 200 xcexcm to 1000 xcexcm and extended along the resonator. The length of the resonator is decided by the pitches of the facets, formed through cleaving, of the resonator of the laser device.
Thirdly, the semiconductor laser device used here is provided with a ridge-shaped waveguide structure, which employs a multiplexed quantum well structure as its emission active layer, more preferably, employs a strained multiplexed quantum well structure as the emission active layer. This strained multiplexed quantum well structure employs lattice distortion.
Details of each of the semiconductor laser devices of the present invention and the optical information processing system that uses such a semiconductor laser device will be described below in the preferred embodiments of the present invention.
These and other objects and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.